Motor's power density

[swbluto]

I was wondering, do electric motors have a higher theoretical power density than gasoline motors?

I've noticed that my R/C motor that regularly propels me to 30 mph without a sweat is similar in result to a 43cc gas scooter, but I typically notice the gas scooter's engine is much larger - it's as large as a 10" wheel, but my RC motor is less than 1/2 the size of my 8" wheel. - it's actually 2 and half inches in diameter and 3 inches long which leads me to think...

Is an electric motor's top practical power density higher than a gasoline engine?

Yes, I know, there are sucky electric motors and sucky gasoline engines but I'm talking about the best of the best of each category.

Theoretically, a gasoline motor's power limit (talking about the best of the best here) is limited by its volume, as a greater amount of heated gas means greater pressure and resulting higher power. An electric's power limit? Whatever doesn't over-heat the motor and burn out the windings (And, ultimately, the magnet's saturation limits). So, theoretically, if you could super-cool the windings so they're effectively superconductors, then your power is limited by the RPM and the point at which the magnets are saturated. Besides the magnet strength and the size of the magnet, it doesn't sound like it's as directly related to volume as a gasoline engine. So, that makes me think an electric's theoretical power density is greater than a gas's. But, on the other hand, the top speed for a gasoline vehicle is much higher than for electric vehicles suggesting much more power is available at the extreme of performance - is there another limit that plays into that besides volumetric considerations (excluding the requisite volume for carrying the batteries) ?

 

[jag]

I think one has to differentiate on type of motor. There are RC motors that produce several hp, yet weigh less than a kg. There are also very light high power RC gas engine. I would suspect that neither last long compared to other motor types. On the other end of the reliability spectrum there are industrial motors designed for running 10,000, 100 000 or more hours in either gas or electric. These can be 100 times the weight of an RC motor for the same power.

In the category of motors for vehicle propulsion, it seems like the electric motors are somewhat lighter than the gasoline motors they replace, but not dramatically (maybe a factor 2-4).

 

[swbluto]

I did note "the best of the best", and I'm talking about motors/engines that approach infinite power (really meaning, the most powerful that's theoretically possible) so you can get an idea of a motor's/engine's asymptotic power density. My puny RC motor was just a small fry that initiated the question with a basic comparison.

But I should've divided by gravimetric density and volumetric density. I guess I'm more curious about volumetric density (watt/m^3).

 

[spinningmagnets]

I would say that if you are talking about common-construction engines and motors, the motors win by a large margin.

Concerning range, gasoline/diesel are much more energy dense per volume than Lithium batteries. But as far as the engine...You can cram up to twice as much air/fuel than natuarally aspirated by using a turbo or blower, but thats about the limit where heat shedding become a big problem.

Common-construction engines experience valve-float around 9,000-RPM for a V8 377-ci. Smaller pistons and valves are lighter, so some small motorcycle engines easily run to 12,000-RPM's. F1 race cars run around 18,000-RPM's, but they use exotic pneumatic systems to open and then close the valves.

For electric motors, if you add active cooling (such as a fan) and use very high volts to keep amp-heat below the damage thresh-hold...I am not as expeienced as most of the other posters here, but I'm certain motors easily provide more power per volume. Consider the biggest RC motor available now (80mm dia-100mm long?) and run at 72V with a fan, compared to a 50cc engine about the same size at 4,000-RPM putting out roughly 2-HP/1.5kW...recently recumpence mentioned a 4,000W RC-build, and then wanting to do one bigger!

 

[Doctorbass]

Swbluto, You should consider calculating the RPM ratio...

a RC motor have high RPM and sometime it is necessary to use grearbox.. that add weight & volume...

I know you dont like hub motor 8) but.. hub motor often dont need any graerbox or chain or belt.. RC motor need that..

Doc

 

[swbluto]

Swbluto, You should consider calculating the RPM ratio...

a RC motor have high RPM and sometime it is necessary to use grearbox.. that add weight & volume...

I know you dont like hub motor 8) but.. hub motor often dont need any graerbox or chain or belt.. RC motor need that..

Doc

So do gasoline engines, Doc, and that's what being compared here. As far as I know, RC motors don't need any more of a gear ratio than gasoline motors, and with larger motors, it can be much less. This is more of a theoretical/practical limitation type of discussion, not necessarily a practical, most convenient e-bike solution comparison. I reserve that for my "I don't like hub motors" thread.

(But, by the ways, I've found some geared hub motors seem to be just as excellent as RC motors for more moderate power levels, so there's a sub-class of ebike hubs that I do like - albeit, a bit more expensive, especially at the financial power density level (watt/dollar). )

 

[set]

swbluto, when you say the best of the best, does that include full size car engines? Does that also include turbo-charged/supercharged engines?

 

[swbluto]

swbluto, when you say the best of the best, does that include full size car engines? Does that also include turbo-charged/supercharged engines?

Yes to the yes! Everything! But, of course, you have to add in the "super-chargers"/"turbo-chargers" to the mass/volume count and that may not necessarily result in the best volume or mass power density (I'm just leaving open the possibility given my ignorance.).

 

[set]

The Mazda wankel rotary 787B is the strongest naturally aspirated engine that I know. It is a 3.5 liter non-turbo engine that produced 700 hp at 9,000 RPM but Mazda claims it had a maximum of 930 hp at 10,500 RPM. If you look at the engine and compare it to the respected Honda VTEC engines. The Vtec engines can produce about 100 hp per liter. Using the 700 hp benchmark, the 787B produced 200 hp per liter. Using the 930 hp benchmark, the engine produced a little over 265 hp per liter.

The car was used at the LeMans 24 hour race in 1991. Out of the history of the LeMans, Japan was never able to produce a win until that race. The next year, LeMans changed the rules banning rotary engines to participate.

Turbocharged cars produced tons of power. Some can produced well over 1,000 hp but that changes every day and I don't know the "best" off the top of my head.

 

[bearing]

Turbocharged motorcycle engines have produced more than 800 hp from 1.3 litre. I think those engines with gearbox are about 80 kg, so thats 10 hp / kg. I don't know the volume. Seems unfair to compare engines with gearboxes to motors without though.

 

[swbluto]

Turbocharged motorcycle engines have produced more than 800 hp from 1.3 litre. I think those engines with gearbox are about 80 kg, so thats 10 hp / kg. I don't know the volume. Seems unfair to compare engines with gearboxes to motors without though.

I don't think the gearbox is necessary for the engine to output power. The gearbox doesn't really add power (in fact, it takes it away!), it just converts the angular velocity and torque to something useful in vehicular applications. This is more like "gasoline engines" vs. "Electric motors" of all application types, the best of the best.

So, anyways, stripping that away seems to be about 12 hp / kg? That seems impressive. The best R/C motor(The plettenberg predator) seems to put out 12 kW for 1.5 kg which is about 10.6666 Hp/kg, but I don't really know what that thing's "true peak" is since an electric motor can exceed the rated power by quite a bit but only for a short moment of time. And, also, these turbocharged engines do you use water-cooling, don't they? That seems like it might be unfair to compare an air-cooled motor to an water-cooled engine...

Anyways, I'm just curious if electrics have some fundamental power advantages to gasoline, regardless of the current battery technology that may be holding it back. If this is so, then it seems inevitable that electric vehicles will surpass gasoline vehicles at the highest levels of performance as battery/energy-storage tech satisfactorily improves.

 

[set]

I think this thread is missing something important. Torque. You can't deny that electric motors generally exceed gasoline engines in that category. HP gets you max speed. Torque gets you there faster.

 

[Link]

I read somewhere that a certain kind of motor (I think a pancake PMAC) has triple the theoretical power density limit of a gasoline engine. Something like 9kW/cc vs. 3kW/cc, I think? :?

Not the slightest on how they came up with either of those limits, though, so don't ask.

 

[TylerDurden]

It might be worth defining "gas" as ICE...

 

[bearing]

I think this thread is missing something important. Torque. You can't deny that electric motors generally exceed gasoline engines in that category. HP gets you max speed. Torque gets you there faster.

Torque doesn't do any work, only power does, with time. If two motors with equal peak power have a linear power curve but one of them have peak power at twice the RPM, their torque will differ by a factor of two. If fitted on equal vehicles and geared to the same top speed they will both get there in equal time, even though one of the motors got half the torque.

I assume you are talking about high torque at low speed, which is the same as high power at low speed. That will of course accelerate a vehicle faster than an engine with equal peak power but lower power at low speeds.

 

[deronmoped]

Here is something to chew on.

A Top Fuel dragster can consume 23 gallons of nitromethane in a run (that is burnout and everything). The engine produces somewhere between 7,000 to 8,500 HP, just the exhaust gases alone produce 800-1000 pounds of downforce.

The internal combustion engine keeps producing more and more power the more air/fuel you can stuff into the cylinder, that is, till it explodes.

Deron.

 

[liveforphysics]

When I was into fuel rc cars, I had rc car engines that weighed 350gr and made 5.5hp, and would make 5.5bhp constantly for like 20mins continously on a fuel tank the size of a teacup. I don't know of any electric motors that do that.

The 787b was not a 3.5L engine no matter how you want to measure it. Each rotor has 3 659cc chambers in each rotor. At any given point, it has no less than 7.9L of gasses acting inside the engine, and the quanity of air it draws through it is no less than 5.2L per cycle. Only through marketing that tries to play on peoples ignorance are they able to claim to be 1/3rd of there actual displacement. They are the least fuel efficient and most knock prone style of mass production ICE.

At Endyn (energy dynamics labs) in Texas where I did an intership in college, they have a tiny turbo charged 100cc methonal burning 2 stroke kart engine. Its air cooled, and with the turbo, the whole package isn't over 30lbs. It makes a whisker over 100bhp. That's 1,000bhp/L. This engine was also thrown together in the late 70s... Far from modern technology.

KTM makes dirtbike engines for kiddy dirtbike racing classes that are 50cc, and 18bhp. They are made to take increadible abuse and run for 100s of hours between rebuilds.

You are right about Honda vtec engines being some of the very best, but as far as vtec honda engines maxing at 100hp/L...... JDM F20c does 125bhp/L stock, and my endurance road racing civic on pump gas makes over 150bhp/L. Drag racing naturally aspirated K20A honda engines often reach 175bhp/L, or 200bhp/L on methonal. Turbocharged, 400bhp/L is not at all uncommon for them.

 

[spinningmagnets]

With ICE, fuel is cheap, and high-output/tiny-engine is expensive...

With E-motors, lithium batteries are expensive, but an Astro at 72V/100A (7kW/9-HP?) is suprisingly affordable...

"Affordable" compared to a custom-built tiny turbocharger with associated oil-pump, oil-cooler. Astro is now off-the-shelf availability, compact high-output ICE would be an expensive time-consuming custom build, and again, very expensive.

There are some interesting small UAV engines, like tiny air-cooled diesel wankels, no reason you couldn't add a belt-driven impellor. But very expensive, and you can't just order one. There are low-tech 32cc, 50cc, 70cc engines that can be ordered and bolted right on. Also worth considering, when an ultra-cutting-edge ICE is running near the ragged edge of max possible output...how long would you expect it to last...compared to an electric motor?

 

[TylerDurden]

^Agree^

Higher power density in Emotors requires much fewer fancy components than boosting ICE motors; just higher quality versions of the typical bits.

 

[liveforphysics]

Doesn't need to require fancy boosted parts and things.
The engine in my first example was a 30,000rpm nitro burning RC car engine. It lasted me many hours of playing and never failed, but I know it wouldn't hold up for E-bike duty, and the RPMs are inconvenient high for powering something with large wheel diameter. It's an engine purpose built for packing as much power as possible onto a little 4lbs RC car.

Doesn't need to be a high strung nitro burning super high RPM engine to be light and powerful. There are some that are pretty dang cheap, and extremely reliable. Like, into the thousand hours between rebuilds type of reliable, and the rebuilds only cost about $50 in parts and take an hour to do.

Something like this runs off regular gasoline, weighs 5.6lbs, and will maintain 11.2bhp for however long you keep fuel in the tank. The RPM range is very friendly for E-bikes, and it's a very compact little package. Much smaller and cheaper and lighter to setup than a 10hp electric motor, and a 10hp electric motor only runs for 8minutes on a 1kwhr battery, where this little guy will run for hours and hours on a little fuel tank.

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7503&Product_Name=DLE-111_111cc_Gas_Engine_11.2HP/7500RPM

However, electric is very quiet, and no emissions, and you can count your bike as a pedal bicycle, which are huge advantages over ICE's.